Update 5 more importable resources' docs

[rileykarson]

Update the documentation of 5 more of our importable resources to include Import blocks specifying how to import them.

We now have explicit documentation for:

• google_compute_forwarding_rule
• google_compute_global_address
• google_compute_http_health_check
• google_compute_instance_group_manager
• google_compute_instance_template

As in #119, to review:

In each Import block;

• The fields specified as being used to import a resource should match their names in the schema exactly.
• The resource address and properties used to perform the example import should be the values used in the example.
• The properties used to perform an import should not be ambiguous; they should not share the same name, for example.

[stack72]

LGTM! thanks for this :)

[ghost]

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